Voltage clamp techniques allow direct control of the membrane voltage of cardiac muscle cells. These techniques are particularly useful in analyzing the ionic basis of the cardiac action potential and excitation-contraction coupling in heart muscle. I propose to use the two-microelectrode voltage clamp technique to examine the relations between membrane voltage, membrane ionic currents, and contraction in the sheep cardiac Purkinje fiber. The principal goal for the project is analysis of the slow inward current (Isi) and its relation to contraction. Isi appears to be directly linked to contraction, and slowly conducted responses arising from the Isi system have been suggested as a cause of arrhythmias. Little is known of Isi in Purkinje fibers because of the difficulty of measuring this component of the net current, in the presence of other currents which distort it or mask it completely. The largest simple competing current appears to be eliminated if fibers are bathed in a low chloride solution containing 0.5 mM 4-aminopyridine, and a deflection of the net current record which resembles the presumed time course of Isi can be seen over the full physiological range of voltages. Experiments are planned to confirm that Isi is revealed by this treatment, and to analyze the kinetics of Isi. The relation of Isi kinetics to the kinetics of contraction will be determined, and we will try to see if there is a correlation between Isi and restitution of contraction. Isi will also be examined under conditions that produce after contractions, oscillations of restitution, and transient inward currents like those record in acetylstrophanthidin, to see if these are related to changes in intracellular calcium resulting from current carried by Isi. These experiments should provide information on the cellular basis of triggered activity in heart muscle.